1. Field of the Invention
The present application pertains to integrated circuit amplifier circuits and more particularly, to compensation techniques for ensuring stability of CMOS line driver and large signal operational amplifier circuits.
2. State of the Art
The integrated operational amplifier was introduced in the early 1970's, and has become commonplace in modern electronics. These amplifiers typically have a differential input amplifier, conversion circuitry that produces a single-ended signal from the output of the differential input stage, in many cases a second gain stage, and an output driver. While these amplifiers were originally fabricated with bipolar technology, they are now frequently fabricated in CMOS processes. These amplifiers are often combined in a system with a feedback circuit connected between the amplifier output and the amplifier input, where the feedback circuit provides a negative feedback to the amplifier inputs and controls the overall transfer function of the system.
Amplifier circuits having two or more gain stages and having negative feedback around the amplifier (or operated in a closed loop) may become unstable or marginally stable with severe ringing. If the loop gain (the gain of the amplifier times the transfer function of the feedback circuit) is greater than one at the frequency where the feedback signal has a 180-degree phase shift, the feedback signal will regenerate itself and the circuit becomes unstable. If the loop gain at this frequency is close to one, undesirable ringing may occur.
A typical two-stage amplifier without output buffering has an open loop transfer function with two poles: ##EQU1## where GMi and GMo represent the transconductance of the first and second stages, R1 and C1 are the resistance and capacitance that load the first stage, and Ro and Co are the resistance and capacitance that load the second stage. With feedback B(s), a loop gain of A(s) x B(s) is obtained, where the loop gain has at least two poles. Generally, these poles occur at different frequencies. The feedback signal asymptotically approaches its first 90 degrees of phase shift at a frequency a decade above the first, lowest frequency or dominant pole, and its second 90 degrees of phase shift at a frequency a decade above the second pole.
To ensure stability at all closed loop conditions, an amplifier must be compensated so that the unity gain bandwidth occurs at a frequency not substantially above the frequency of the second pole. This is typically done by adding a compensation capacitance to move the dominant pole of the transfer function to a lower frequency. Assuming that the first pole is the dominant pole, and that the frequency of the second pole is greater than the unity gain bandwidth, the gain bandwidth product (in radians) of the compensated amplifier is approximated by: ##EQU2## where Ctot is the total effective compensation capacitance that loads the first stage plus C1.
In amplifier configurations with a non-buffered output where the current in the output stage is proportional to the output signal level, such as a class AB output stage driving a resistive load, the output stage gain varies with output signal level. This is because the gm (transconductance) of the MOS output devices changes as the square root of the current in those devices. The transconductance of bipolar output transistors changes proportional to output transistor current. When output currents are large, the amplifier gain is larger than when the amplifier is quiescently driving a very low current. An output stage having these characteristics is described by Babanezhad in U.S. Pat. No. 5,157,349, issued in October 1992.
The loop gain, the gain of the amplifier and the associated feedback circuitry combined, increases with the gain of the amplifier. Variation of the amplifier gain with signal level therefore complicates the compensation--if the amplifier is compensated properly for its largest output signal level, then it will have unduly limited bandwidth at the quiescent point. A compensation technique which can maintain a relatively constant unity gain bandwidth at all output voltages should be used.
Compensation of integrated operational amplifiers is often accomplished by adding a capacitor from the input to the output of the output stage of the amplifier. This takes advantage of the Miller multiplication effect to increase the effective capacitance of the capacitor. The capacitance moves the dominant pole to lower frequency and the second pole to a higher frequency resulting in better phase margin than the amplifier would have without the capacitor. In this scheme, the Miller effect allows a small capacitor (Cc) to be used because the effective capacitance (Ceff) is approximately equal to the voltage gain of the output stage times the capacitor's actual value.
Equation 3: EQU Ceff=Cc X(1+GMo.times.Ro)
In this equation, GMo corresponds to the gm of the output device and Ro is the resistance seen by the output stage. Since the loop gain depends on the gain at this stage (GMo.times.Ro), any change in the gain will affect both the loop gain and the amount of compensation. For example, as the output signal increases, the current in the output stage increases, and the gain increases. This gain increase is accompanied by an increase in the effective capacitance which in turn shifts the dominant pole of the compensated amplifier to a lower frequency. The overall gain bandwidth will remain relatively constant except at very low output levels when the gain of the output stage is near or less than one.
In cases where the second stage uses large devices to accommodate large output currents, compensation using Miller capacitance becomes difficult. The large devices introduce large parasitic capacitances. At low bias current the output stage gain is low, with the result that the Miller multiplied feedback capacitance is small compared to the input capacitance of the output stage and thus ineffective at keeping a constant gain bandwidth product for the amplifier until the output stage gain reaches a large value. A large feedback capacitance can be used but more input transconductance is necessary to achieve a desired gain bandwidth product.
In integrated circuits, large capacitances occupy a large area of the die, require extra processing steps, or both. Circuit cost increases with the die area and with the number of processing steps, therefore either of these increases the cost of the circuit. Also, the larger required transconductance results in more power consumption and larger area. It is therefore desirable to compensate a high power amplifier with an alternative method which maintains a constant unity gain bandwidth and allowed large output swings but is more compact than Miller compensation as heretofore practiced.